The present invention relates to a reflective liquid-crystal display device with a reflecting electrode and more specifically to a liquid-crystal display device which provides improvements in the reflecting structure.
In recent years, the development of new types of display devices that will replace conventional cathode ray tubes has been becoming active. In the market for household electrical appliances and OA equipment, great expectations are held especially for liquid-crystal display devices because of their flatness and low power dissipation.
Heretofore, principal liquid-crystal display devices are of the so-called transmissive types in which a flat type of light called backlight is put on the back of the liquid-crystal panel. However, relatively large power dissipation is produced by the backlight, which constitutes a main cause to prevent the low power operation that should be a merit inherent in liquid-crystal display devices.
Thus, a possible way will be to use a reflective liquid-crystal display device, in which a reflecting plate is placed at the back of the liquid-crystal panel to reflect surrounding light back forward. With this device, there is no need of backlight, ensuring very low power dissipation.
With the reflective liquid-crystal display device, however, the transmittance of the liquid-crystal panel is low, say, a few percent to a few tens of percent. Thus, the reflection of surrounding light alone fails to obtain a satisfactory brightness. For this reason, the reflective liquid-crystal display devices have not been put to practical use except specific applications to wrist watches, hand-held calculators, and the like. However, with the recent development of hand-held equipment, low-power display devices are in increasing demand and hence reflective liquid-crystal display devices are becoming reconsidered.
The point of the reflective liquid-crystal devices is brightness, i.e., reflectance. The reason is that poor transmittance of the liquid crystal must be compensated for by increasing the reflectance to increase visibility. To attain high reflectance, a high-performance reflecting plate is needed.
The reflecting plate should preferably have a property of diffusing light perfectly as shown in FIG. 1. To compensate for poor transmittance of liquid crystal, one possible way will be to use a reflecting surface having a property of reflecting light strongly in specific directions. In this case, although the viewing angle is narrowed down, it is possible to obtain reflection stronger than perfect diffuse reflection with respect to the specific direction.
A method has been proposed which uses a reflecting electrode having a mirror surface as such a reflecting surface. In this case, the reflection in the direction of specular reflection is very strong. However, a problem arises in the use of a mirror surface in a liquid-crystal display device as a reflecting surface in that, when viewed from an angle displaced by a fraction from the direction of specular reflection from the mirror surface, the reflection strength will become virtually zero.
In addition, as shown in FIG. 2, the direction of surface reflection from a front panel 1 of a liquid-crystal display device and the direction of reflection from an reflecting electrode 6 coincide with each other, so that the display screen is viewed shining white, considerably decreasing contrast. In FIG. 2, reference numeral 2 denotes a liquid-crystal layer, 3 an electrode opposed to the reflecting electrode 6, and 7 a substrate.
In a TN (twisted nematic) type of liquid crystal that combines light transmitted through the liquid crystal with polarizers, reflected light from the mirror surface has an effect of increasing image contrast. In a guest-host type of liquid crystal that utilizes light absorption by dye, however, satisfactory coloring cannot be attained and satisfactory contrast cannot be realized.